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Active Lecture PowerPoint® Presentation for
Essentials of Genetics
Seventh Edition
Klug, Cummings, Spencer, Palladino
Chapter 23
Population and Evolutionary
Genetics
Copyright © 2010 Pearson Education, Inc.
How traits
are passed
from one
generation to
the next
Genetic compositions of
groups/populations - how
they change and why…
The nature
of genes
Population Genetics
• Study of genetic variation in groups or
populations and how it changes over time.
• A population is a group of individuals with
a common set of genes that lives in same
geographic area and interbreeds
23.1 Genetic variation is present
in most populations & species
• This variation reflects the variation in the
alleles distributed among populations of a
species
• A population's gene pool is all of the
alleles present in that population
• Due to population dynamics, the gene pool
can change over time
Genetic variation in Lady-bird beetles
Copyright © 2010 Pearson Education, Inc.
Detecting genetic variation by
artificial selection
• Dog example
• All dogs are descendents from wolves
• Dramatic change in phenotype occurred in about 15,000
years.
• Broad array of sizes, shapes, colors and behaviors seen
in dogs all arose from variation present in wild wolves.
Figure 23-1 Alcohol
dehydrogenase
(Adh) gene in Drosophylla
Copyright © 2010 Pearson Education, Inc.
Figure 23-2 DNA sequence
variation
in Adh gene in Drosophylla
Copyright © 2010 Pearson Education, Inc.
23.2 The Hardy–Weinberg Law
Describes what happens to alleles and genotypes
in an “ideal” population assuming
1.
2.
3.
4.
5.
No selection
No mutation
No migration
Large population
Random mating
The Hardy–Weinberg Laws
1. Frequency of alleles in the gene pool does
not change over time
2. After one generation of random mating,
genotype frequencies for two alleles can be
calculated as
p2 + 2pq + q2 = 1
where p = frequency of allele A
and q = frequency of allele a
Allele vs. Genotype Frequency
•
Alleles
Freq of allele A =0.7 freq of allele a =0.3
•
Genotypes
AA, Aa, aa
•
HW law explains the relationship between
allele and genotype frequency in a
population
Allele frequencies after one
generation under HW Law
•
Parental generation
Freq of allele A =0.7 freq of allele a =0.3
•
How do you calculate allele frequencies after
one generation?
Allele frequencies in the next
generation
AA - 49%
Aa - 42%
aa - 9%
Frequency of allele A = 0.49 + 0.21 = 0.70
Frequency of allele a = 0.09 + 0.21 = 0.30
Allele frequency has not changed from the
previous generation
The Hardy–Weinberg Law
• The Hardy-Weinberg law allows the
frequency of heterozygotes in a population
to be estimated
• In general, the frequencies of all three
genotypes can be estimated once frequency
of either allele is known and HardyWeinberg assumptions are invoked
The Hardy–Weinberg Law
• Problem 1:
Human blood types, MM, MN and NN are
controlled by two alleles (M & N) of a single gene.
In a population of 100 individuals, 49% are of the
NN blood type.
What percentage is expected to be MN assuming
Hardy-Weinberg equilibrium conditions?
The Hardy–Weinberg Law
• Problem 2:
In a population of 100 that meets the HardyWeinberg equilibrium assumptions, 81 individuals
are homozygous for a recessive allele.
What percentage of the individuals would be
expected to be homozygous for the dominant
allele in the next generation?
The Hardy–Weinberg Law Assumes
1. There is no selection
2. That no new alleles arise from mutation
3. That there is no migration into or out of
the population
4. That the population is infinitely large
5. That random mating occurs
Change in allele frequency is
evolution!!!!
23.3 The Hardy–Weinberg Law can be
applied to human populations
•
CCR5 gene – Chemokine co-receptor 5
•
Normal allele –CCR5-1
•
Δ32 mutation – 32 bp deletion in the gene
•
In a population of 100 people
–
Homozygous for CCR5 - 1/1
–
Heterozygous
-1/ Δ32
–
Homozygous for Δ32
- Δ32/ Δ32
79
29
1
Forces Driving Allele Frequency
Change (p 491)
• Natural selection
• Mutation
• Migration
• Genetic drift in small populations
• Non-random mating
Natural Selection
• Natural selection takes place when
individuals with adaptive trait produce a
greater number of offspring than that
produced by others in a population
• In the next generation, the offspring with
the adaptive trait appear in greater
frequency
• What are some examples?
Natural Selection
• If individuals are subject to natural selection
and do not have equal rates of survival and
reproductive success, allele frequencies
may change from one generation to the
next
• Natural selection is the principal force that
shifts allele frequencies within large
populations
1 Population with varied inherited traits
2 Elimination of individuals with certain traits
3 Reproduction of survivors
Natural selection
acts on polygenic
traits too
Mutation
• Mutation is the only process that creates
new alleles in a gene pool
• Mutation creates new alleles on which
natural selection acts upon
• Change in allele frequency due to
mutation depends on the fitness they
confer and the action of natural selection
Migration or Gene Flow
• Migration occurs when individuals move
between the populations
• Migration may have a large effect on allele
frequency if:
– the rate of migration is large
or
– if the allele frequency of the migrant population
differs greatly from that of the population to
which it is moving
Migration – B allele of ABO locus
Genetic Drift
• Genetic drift occurs when # of reproducing
individuals in a population is too small to
ensure all alleles in the gene pool will be
passed on to next generation in their
existing frequencies
• Genetic drift may result in one allele
becoming fixed and one allele
disappearing in a population
Nonrandom Mating
• Nonrandom mating can change the
genotype frequency in a population
• But not the allele frequency
Forms of Nonrandom Mating
• Positive assortive mating in which similar
genotypes are more likely to mate than
dissimilar ones
• Negative assortive mating in which dissimilar
genotypes are more likely to mate than
similar ones
• Inbreeding in which mating individuals are
related
Inbreeding
• One consequence of inbreeding is an
increased chance that an individual will be
homozygous for a recessive deleterious
allele
• The significance of this fact is that inbred
populations often have a lowered mean
fitness, called inbreeding depression
Inbreeding in plants
• Self-fertilization is a form of inbreeding
• Continuous self-fertilization can lead to fully
homozygous plants called inbred lines
• If members of two inbred lines are mated,
the offspring often display hybrid vigor.
• Hybrid vigor is highest in the F1 generation
and generally declines thereafter.
Inbreeding: First-cousin marriage